Input range selection USB 6251

[Cnna]

Hello. I am using a USB 6251 board to read some voltages. Initially I thought that I need to select the input range in function of the voltage what I would like to read (for ex. for 4V DC the best range is +/-5V and not +/-10), if I want to obtain as precise results as possible.

But when I connected 500-600uV to the input, it seemed that the best range is +/-2V. The smaller ranges cut a part of the signal, while the higher ranges display much higher voltage, even up to 1-2mV.

Could anyone explain me, why is this working like this? Why shouldn't I use the +/-0.1 V range for such small voltage? Does this depend on the sensitivity of each range? 

Thank you very much! 

[Jackie_B]

Hi Cnna,

From your post, it sounds like you are doing an analog input task. If this is the case, we need to confirm the exact range of analog input voltages you are reading with a voltmeter. The supported analog input range for the USB 6251 is ±10 V, ±5 V, ±2 V, ±1 V, ±0.5 V, ±0.2 V, ±0.1 V, so for 500mV, 0.0005 volts, you should be using the +/-0.1 V range. The fact that you are seeing ranges of 1-2mV instead of 500uV indicates that the signal you are reading may be an unexpected voltage.

If you are doing DIO (which is indicated when you say 4V DC) please be aware that for our DAQ cards, the range for DIO is 0-5V, 5 being high and 0 being low.

Please confirm that you are doing an analog input task and that your signal is 500-600mV

Regards,

Jackie B

Applications Engineer

[johnsold]

Jackie B,

Please be careful about your units.  You used mV (= millivolts) where it appears that you meant uV or microvolts.

Cnna,

What is the source of your microvolt signals and how are you connecting them to the USB-6251?

I am thinking you could easily have ground loop or thermoelectric potentials causing your mV offsets.

Lynn

[Cnna]

Thank you for the answers!

I try to measure the voltage drop on a resistor. I use differential configuration for the DAQ. In the same time I generate some DC signals on the AO-s of the board and I use them as supplies or input voltages to my circuit. An AO terminal is referenced with respect to AO GND, while the inputs with respect to AI GND. Can this cause ground loop? What if I connect the two GND-s together? 

Thank you!

Cnna. 

[johnsold]

Cnna,

The grounds are probably connected together internally in the device.

What is the common mode voltage on the resistors?

Can you post an image (.png) of your circuit?

Lynn